1. Field
This application relates generally to communication and more specifically, but not exclusively, to the use of multiple pilot signature indicators for identifying an access point.
2. Introduction
A wireless communication network may be deployed over a geographical area to provide various types of services (e.g., voice, data, multimedia services, etc.) to users within that geographical area. In a typical implementation, access points (e.g., macro access points providing macro cell coverage) are distributed throughout a network to provide wireless connectivity for access terminals (e.g., cell phones) that are operating within the geographical area served by the network.
As the demand for high-rate and multimedia data services rapidly grows, there lies a challenge to implement efficient and robust communication systems with enhanced performance. To supplement conventional network access points (e.g., macro access points), small-coverage access points may be deployed (e.g., installed in a user's home) to provide more robust indoor wireless coverage or other coverage for access terminals. Such small-coverage access points may be referred to as, for example, femto access points, femto cells, home NodeBs, home eNodeBs, or access point base stations. Typically, such small-coverage access points are connected to the Internet and the mobile operator's network via a DSL router or a cable modem.
As the access terminal roams throughout the geographical area associated with the network, the access terminal may move away from its serving access point and move closer to another access point. Consequently, when an access terminal gets close to a particular access point, it may be desired to handover (i.e., idle or active handover) the access terminal to that particular access point if that access point provides better radio frequency (RF) coverage and/or additional services.
To enable such handover, access terminals in a network regularly monitor for pilot signals from nearby access points to identify potential target access points. To facilitate this monitoring, each access point transmits a pilot signal with a unique pseudo-random noise (PN) spreading code. Different access points in the network may use a known pilot spreading code (also sometimes known as scrambling code) with different phase offsets—commonly referred to as PN offsets (e.g., for the case of a cdma2000 network). Thus, an access point may be identified based on the PN offset used by that access point. In conventional macro networks, a target access point for handover of an access terminal between two cells is identified based on a forward link (FL) pilot report sent by the access terminal. Such a report may be referred to as, for example, a pilot strength measurement message (PSMM) or as a Route Update (in CDMA high rate packet data technology). The pilot report includes an indication of the FL signal quality (typically pilot strength Ecp/Io) of neighboring access points and pilot phase associated with each of these access points. The pilot phase that is reported may then be mapped to the signature (e.g. pilot PN offset) used by a particular access point. In this way, the identity of the access point that transmitted given pilot signal may be determined assuming no other access points are using the signature.
For effective active (i.e., connected) handover of an access terminal from one access point to another, the network needs to be able to uniquely identify the target access point. However, the number of available PN offsets is typically limited. In some cases, the number of available PN offsets may be limited by the size of the neighbor list that is used to assist access terminals in searching for neighboring PN signals. Here, to reduce overhead and improve efficiency, it may be desirable to limit the number of entries in the neighbor list advertised by a macro access point to a relatively small number (e.g., 20-40).
Consequently, in the event a relatively large number of small-coverage access points are deployed in the same area (e.g., within the coverage of a single macro cell), several of these access points may use the same PN offset for their pilot signals. Unique identification for active handover to such an access point may therefore be difficult due to PN offset confusion. Specifically, confusion may exist as to which access point (e.g., which potential handover target) is being identified when an access terminal in the network reports to its serving access point (e.g., the handover source) that a pilot signal having a given PN offset has been received.
Conventional solutions for dealing with the above problem include a mobile sensing scheme and a scheme where an access point advertises a cell identifier. For example, in a mobile sensing scheme, candidate target femto cells are requested to detect signals from an access terminal on the reverse link (RL) and report this information to the network. The network then identifies the target based on which femto cell reported the best FL signal. In practice, however, such a scheme may have scalability problems in the event a large number of femto cells are deployed. In addition, such a scheme may not provide a sufficient level of accuracy due to FL/RL imbalances (e.g., the femto cell that reports the strongest FL signal may not be the intended target).
In a cell identifier advertising scheme, a femto cell may advertise an access point identification message that includes a mobile switching center (MSC) related identifier (IOS_MSC_ID) and a cell related identifier (IOS_CELL_ID) that uniquely identifies that femto cell at the network. An access terminal may then report this information to the network via a handoff supplementary information notification message. However, such a scheme requires that the macro access points be upgraded to support the handoff supplementary information notification message. In addition such a scheme does not support legacy access terminals. In view of the above, there is a need for effective techniques for identifying access points so that other nodes in the network may efficiently communicate with the access points.